The invention relates to a solution of tetramethylammonium hydroxide in water with a surfactant component, and to a process for preparing a solution for developing an exposed photoresist.
In wafer production, a photoresist mask is applied to the wafer, exposed and then developed using a developer liquid. A conventional developer is based on a 2.5% strength solution of tetramethylammonium hydroxide (TMAH) in water, comprising a small amount of added surfactant. The surfactant is able to reduce the interfacial tension of the developer solution to an appropriate, defined value.
The amount of developer solution required in a wafer factory is considerable: the development of a photoresist mask on a single wafer requires about 50 ml of developer solution. Currently, the production of a finished wafer comprises about 20 lithography steps which means that it is also necessary to carry out 20 development steps. A wafer factory producing 10,000 wafers per week requires approximately 10,000 liters of developer liquid a week. The transport of the developer liquid to the factory thus gives rise to considerable costs.
It has been suggested that a more highly concentrated TMAH solution (for example 25% strength) be transported to the factory and diluted in the factory itself to give the desired final concentration. The deionized, high-purity water required for this purpose is available in all wafer factories. A reduction of transport costs would be achieved by this measure.
There is, however, a practical problem in that the surfactant does not dissolve in such a concentrated developer solution at standard temperature. The surfactant precipitates in the concentrated solution, i.e. it forms insoluble aggregates, which may form deposits on container walls. For this reason it is not possible to transport the surfactant in a concentrated TMAH solution.
It would be possible in principle to transport the developer solution to the factory in concentrated form without surfactant and add the surfactant only after diluting the developer solution to its final use concentration (approximately 2.5%). However, this procedure would involve transporting the surfactant to the factory separately in prediluted form and require an extra addition step in addition to diluting with water, which extra step is risky in terms of purity requirements. However, the essential disadvantage of this method is that is is usually not possible in a wafer factory to monitor and control the amount of admixed surfactant in a sufficiently exact manner. These disadvantages are so serious that this procedure is not used in practice.
The first object is achieved, according to the invention, in that the solution further comprises a hydrotropic component.
The object of the present invention is to provide a developer solution which reduces the costs associated with the development step in wafer production. It is another object of the present invention to provide a process for preparing such a solution.
The first object is achieved according to the invention by the characterizing features of Claim 1.
The solution according to the invention has the advantage that it can be both in a concentrated form which is particularly suitable for transport purposes, and in a ready-to-use, low-concentration form.
In the concentrated form, for example at a TMAH concentration of 5-40% and in particular of 25%, the effect of the hydrotropic component is that the surfactant component is soluble even at these increased TMAH concentrations. It is thus possible to transport the TMAH solution of the invention in its concentrated form, correspondingly reducing the amount of solution to be transported and thus the transport costs.
The novel ready-to-use developer solution, having a TMAH concentration of approximately 2,5%, can be obtained by simple dilution of the concentrated TMAH solution with deionized, high-purity water. It was found that the hydrotropic component does not impair the development behavior of the solution.
The hydrotropic component preferably consists of cumenesulfonic acid, tetramethylammonium cumenesulfonate, tetramethylammonium xylenesulfonate, tetramethylammonium toluenesulfonate, tetramethylammonium phenolsulfonate and/or tetramethylammonium benzenesulfonate.
The second object of the present invention is achieved, according to the invention, by adding a hydrotropic component.
For the preparation of a concentrated novel solution intended for transport, the novel process is characterized in that step b) comprises adding the surfactant component in a concentration above the cloud point and step c) comprises adding the hydrotropic component in an amount sufficient to solubilize the surfactant component added in step b). The surfactant and the hydrotrope can be added in any sequence.
Advantageous embodiments of the invention are specified in the dependent claims.